Electric furnace resistor element



Feb 3, 1942.

J` D. HANAWALT ETAL ELECTRIC FURNACE RESISTOR ELEMENT Filed NOV. 6, 1939 lNvENToRS Joseph 0. emma/7 BY 0f/76 4 MZS L ATTORNEYS Patentedl Feb. v'3, 19.42

' ELECTRIC. FUltNACE RESISTOB ELEMENT Joseph D. Hanawalt and Lorne `A. Matheson, Mldland, Mich.,

assignors to The `Dow Chemical Company, Midland, -Mich., a corporation ofv Application November 6,1039', Serial No. 302,964

4 claims.

This invention relates to an improved construction for electric furnace resistor elements and to furnaces comprising the new elements.

In the operation of electric resistance furnaces for certain metallurgical processes which are conducted at temperatures well above 1000" C. and in which metal vapors are liberated, considerable difficulty is encountered due to failure of the carbon or graphite resistor elements used for heating they furnace. Not only are the resistors subject to direct chemical attack by the metal vapors and accompanying gases, but more particularly, because of the conducting properties of the vapors, destructive electric arcs occur repeatedly between the resistors and other electrically conducting-parts of the furnace. As a result of these occurrences, the useful life of resistor elements in such a furnace is exceedingly short and frequent replacements are required.

The principal object of the present invention,

nectlon with electric power leads I9. The terminal blocks I8 are separated from the shell and supported by gas-tight bushings 20 and 2l, of

which one 20 is of insulating material such as Alundum, vand the other 2| may be either of insulating or more commonly of conducting materiaLdepending upon whether it is desirable to ground a resistor terminal on the shell II. The resistor rod I1 is heldin place within the protection tube I5 at one end thereof by an insulating bushing 22 and at the other end by a similarly shaped bushing`A 23 of either insulating or Apreferably of conducting material, depending then, is to provide an electric furnace resistor element construction which is not subject to the disadvantages mentioned. Another object is to provide an element which is readily replaceable and in addition is free to move longitudinally and thus is not subject to breakage resulting from expansion or contraction of the element with changes in temperature. Other objects will be apparent from the description.

The resistor element construction according to the invention will be explained with reference to the accompanying drawing, in which y Fig. 1 isa vertical section of one form of electric resistance furnace embodying the invention; and

Figs.' 2 and 3 are vertical vsections of furnacesl embodying other forms of the invention.

The furnace shown in Fig. 1 consists of a metal shell I I within which is a graphite furnace chamber l2 supported on refractory bricks I3, the

space between the shell and the chamber beingv filled withelectrically non-conducting4 thermal insulation I4. Material may be charged into the furnace and withdrawn therefrom through suitable openings not `shown. Extending through the furnace chamber I2 is a cylindrical graphite` protection tube I5'which makes firm electrical and mechanical contact with the end walls of the chamber at suitable joints I6. Mounted within the protection tube and spaced therefrom is a cylindrical graphitefresistor rod I1, each end of which fits into a recessed graphite terminal block I8. These terminal blocks, which may be watervcooled, if desired, extend outwardly through openings in the ends of the shell II to make conupon whether the protection tube I5 and the furnace chamber walls I2 in contact therewith are to be grounded on a terminal block I8.

It will be observed that in the construction de-v scribed, the protection tube I5 serves to protect the resistor rod from contact with any gases or vapors which may be in the furnace chamber. Because -of the electrical interconnection of the protection tube With the furnace chamber wall, and also with one end of the resistor if desired,

the tube remains at essentially the same potential as the walls of the' furnace chamber; arcing does not occur. In addition, the protection tube and resistor element are free to slide longitudinally relative to one another and to the walls of the furnace, so that expansion changes are compensated.

In the alternative construction shown in Fig. 2,` the graphite protection tube I5 is secured electricallyv and mechanically to the side wall of the furnace chamber I2 at a single joint I6, and extends into the chamber as a cantilever. Within the tube I5 and spaced therefrom is the resistor rod I1, which in this instance fits into only one terminal block I8 extending 'outwardly through an insulating bushing 20 mounted in the shell I I to an electrical lead I9. The resistor I1 is held `in place and insulated from the protection tube y I5 at its end nearest the shell by an insulating bushing 24. At the otherend of the protection tube, however, the resistor I1 is connected `elecof the resistor.

grounded on the shell Il by a connector bar 21 y trically to the tube by means of a graphite plug 25 which fits snugly into the end of the tube and has a central recess 26 adapted to receive the end 'I'he furnace chamber wall I2 is which extends through the shell to provide a terminalZB for an electrical lead I9. In operation, the electric current passes from the terminal block I8 through theresistor bar I1, and

returns through the plug 25. protection tube I5,

furnace wall I2, and connector 21. I The resistor I1 has an electrical resistance very large in comparison to that of' the remaining parts of the electrical circuit, and hence is, practically speaking, the heating element, the other partsv serving merely as electrical leads.

In another form of the invention shown in Fig. 3, the construction -is very similar to that i in Fig. 2. 'Ihe protection tube extends entirely through the furnace chamber I2, and is secured electrically to the walls thereof at joints `I i. Two resistor rods, insulated from the protection tube by Alundum bushings 24, are used. These rods extend into a graphite plug or bushing 29 mounted securely in the center of the tube I5, and

a furnace shell having mounted therein a furnace chamber having electrically conducting wallsna resistor construction comprising an electrically conducting protection tube extending into the furnace chamber 4and. in electrical contact with a .wall thereof,` a resistor mounted Within the protection tube and spaced therefrom, electrical contact means connecting the protection tubeand the resistor at only. one position along the length of the latter, and means for connecting the' resistor and protection'tube in series across a source of current.

2. In an electric resistance furnace including a furnace chamber having graphite walls, a resistor' construction comprising a graphite pro- 1 tection tube extending into the furnace chamwhich, however, issmall because of the relatively low resistance. There is accordingly very little potential between the electrically conducting furnace walls and any point on the protection tube.

Even in furnaces containing metal vapors at low4 pressures, arcing does not occur to any significant extent. The elements of Figs. ,2 and 3 are easily replaceable. i i

In furnaces, according to the invention which are to be used in metallurgical processes at temperatures above 1000 C., the resistor rods, protection tubes, and furnace chamber walls are ordinarily constructed of some form of carbon,

preferably graphite. However, for operation at lower temperatures, the resistors, tubes, and furnace walls may be made of other electrically conducting materials, and need not all be of the same material.

It will be appreciated that the principle of the invention may be employed in forms other than those illustrated. Thus, more than one resistor may be placed within the protection tube. The resistor construction may be mounted vertically as well as horizontally. The resistor and protection tube may be embedded in recesses in the furnace chamber walls, or the walls themselves, if suitably thick, may be used to form partv or all of the protection tube. Other such variations within the scope of the appended claims will doubtless occur to one skilled in the art.

We are aware that others have previously mounted electric furnace resistor elements within refractory protection tubes, and we make no claims broadly to such structure. l

What we claim is:

1. In an electric resistance furnace includingv ber and mounted securely in a wall thereof in electrical contact with the wall, an insulating bushing fitted into the protection tube at a position near the furnace wall and a graphite bushing tted into the tube at a position remote from the insulating bushing, a graphite resistor rod within the protection tube, being spaced therefrom and mounted firmly in said bushings, and means for connecting the resistor and protection tube in series across a source of current.

3. In an electric resistance furnace including a furnace shell having mounted 4therein a furnace vchamber having carbon walls, a resistor construction comprising a carbon protection tube extending into the furnace chamber and mounted securely in 'a wall thereof in electrical contact with the wall, an insulating bushing tted into the protection tube at a position near the furnace wall and a` carbon bushing fitted intothe tube at a position remote from the insulating bushing, a carbon resistor rod within the protection tube, being spaced therefrom and mounted firmly in said bushings, and means for series across a source of current.

4. In an electric resistance furnace including a furnace shell having mounted therein a furnace chamber having electrically conducting walls, a resistor construction comprising an electrically conducting protection tube extending into the furnace chamber and in electrical contact with a wall thereof, a resistor mounted within the protection tube and spaced therefrom, electrical contactmeans connecting the protection tube and the resistor at only one position along the length of the latter, and electric terminals se-V cured one to the resistor and the other to the furnace chamber wall for connecting the resistor and protection tube in'series across asource of current.

JOSEPH 'D. HANAWALT. LORNE-A. MATHESON. 

